The basic CIS based thin-film solar cell module proposed by the present applicants already came up to the IEC 61646 (International Electrotechnical Commission Standards 61646) (first edition). It is generally known that CIS based thin-film solar cell modules temporarily deteriorate in conversion efficiency through the 1,000-hour damp heat test as provided for in IEC 61646 (first edition) (test in which a sample is stored in the dark for 1,000 hours under the relatively high-temperature high-humidity conditions of a temperature of 85° C. and a relative humidity of 85%) but recover their conversion efficiency by light soaking after the test.
The durability test for a CIS based thin-film solar cell module is conducted by the method according to IEC 61646 (first edition). With respect to individual test items, however, tests are made according to solar-cell-related provisions in JIS (see non-patent document 1), for example, JIS C 8911:1998 (secondary reference crystalline solar cells), JIS C 8912:1998 (solar simulators for examining crystalline solar cells), JIS C 8913:1998 (method of measuring output of crystalline solar cell), and JIS C 8914:1998 (method of measuring output of crystalline solar cell module).    Non-Patent Document 1: Taiyō Denchi Kanren JIS Mokuroku (published by Japanese Standards Association)
There actually is a report that a CIS based thin-film solar cell module which has undergone the damp heat test recovers its performance to at least 95% of the initial conversion efficiency upon irradiation with an intense light at an irradiance of 1,000 W/m2 for a period of from 50 minutes to 300 minutes.
At present, the mechanism by which a CIS based thin-film solar cell module recovers by light soaking from the temporary deterioration resulting from the damp heat test has not been elucidated. This is one of the subjects left concerning the CIS based thin-film solar cell module technology.
In order to understand such a phenomenon, the applicants have made investigations on durability tests for CIS based thin-film solar cell modules based on the expectation that a CIS based thin-film solar cell module does not show any deterioration at all when subjected to a damp heat test while continuously irradiating the module with a light for a given time period throughout the test.
As stated above, CIS based thin-film solar cell modules have the following property. Although the modules show temporary deterioration in conversion efficiency after the conventional damp heat test in the dark for 1,000-hours, they recover the original conversion efficiency by light soaking after the test. There has been a problem that the durability of CIS based thin-film solar cell modules is not correctly evaluated by the conventional method of damp heat test.